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The Long-Lasting Blues: A New Record for Phosphorescent Organic Light-Emitting Diodes

2017; Elsevier BV; Volume: 3; Issue: 3 Linguagem: Inglês

10.1016/j.chempr.2017.08.013

2451-9308

Kent O. Kirlikovali, Alexander M. Spokoyny,

Organic Electronics and Photovoltaics

In this issue of Chem, Wong et al. report a blue Ir(III)-based phosphorescent organic light-emitting diode (PHOLED) material with high efficiency and an operational lifetime of more than 2,200 hr in a working device. These characteristics make this material one of the most robust blue PHOLEDs reported to date. In this issue of Chem, Wong et al. report a blue Ir(III)-based phosphorescent organic light-emitting diode (PHOLED) material with high efficiency and an operational lifetime of more than 2,200 hr in a working device. These characteristics make this material one of the most robust blue PHOLEDs reported to date. Thomas Edison revolutionized lighting in the late 19th century with the invention of the first commercially practical incandescent light bulb, a considerable improvement on the oil lamps and candles that were used up until that time. Incandescent bulbs generate light by passing electricity through a wire filament until it is hot enough to radiate visible light; as a result, more than 95% of the energy is lost as heat, and a majority of the light emitted is in the invisible infrared region.1Nobel Media AB (2014). The Nobel Prize in Physics 2014. Nobelprize.org, http://www.nobelprize.org/nobel_prizes/physics/laureates/2014/popular.html.Google Scholar Fluorescent light bulbs, up to five times more efficient and an order of magnitude longer lasting than incandescent bulbs, have been favored over incandescent bulbs since their commercialization in the early 1940s.1Nobel Media AB (2014). The Nobel Prize in Physics 2014. Nobelprize.org, http://www.nobelprize.org/nobel_prizes/physics/laureates/2014/popular.html.Google Scholar Applying an electric current through a fluorescent bulb excites mercury vapor, which then emits ultraviolet (UV) radiation that hits a phosphor coating, ultimately resulting in the emission of visible light. Significant drawbacks include (1) the use of mercury, which presents a health hazard and complicates safe disposal procedures, and (2) the fundamental limitation of luminous efficacy to 90 lm/W,1Nobel Media AB (2014). The Nobel Prize in Physics 2014. Nobelprize.org, http://www.nobelprize.org/nobel_prizes/physics/laureates/2014/popular.html.Google Scholar which is due to the intrinsic loss of energy when UV photons are converted to visible photons (commercial fluorescent bulbs produce about 70 lm/W). Until the development of white-light-emitting diodes (LEDs), fluorescent bulbs remained the most efficient sources of light available. LEDs are currently the most efficient sources of light—they have luminous efficacy values exceeding 300 lm/W and lifetimes (∼105 hr) that are orders of magnitude longer than those of fluorescent and incandescent bulbs.1Nobel Media AB (2014). The Nobel Prize in Physics 2014. Nobelprize.org, http://www.nobelprize.org/nobel_prizes/physics/laureates/2014/popular.html.Google Scholar In LEDs, multiple layers of carefully engineered inorganic semiconductors directly convert electricity into photons; the energy of the photon, and therefore the color of light emitted, can be tuned by variations in the band gap of the semiconductor. To produce white light, one can use a blue LED to excite a phosphor, which then produces the remaining portion of the visible spectrum (similarly to fluorescent bulbs), or combine red, green, and blue LEDs in a single device. Given that lighting accounts for 20%–30% of the world's energy consumption, mass production of white LEDs has significantly contributed to worldwide energy conservation. In addition to more efficient solid-state lighting, larger and brighter lighting panels, screens, and displays have also resulted from the continued improvement in the efficiency of LEDs.1Nobel Media AB (2014). The Nobel Prize in Physics 2014. Nobelprize.org, http://www.nobelprize.org/nobel_prizes/physics/laureates/2014/popular.html.Google Scholar Organic light-emitting diodes (OLEDs) do not rely on inorganic semiconductors but rather on thin layers of small organic molecules (tens to hundreds of nanometers thick) sandwiched between metal electrodes. OLEDs have multiple advantages over their inorganic counterparts: (1) they are orders of magnitude thinner and lighter, which enables the development of transparent devices; (2) they can be applied to flexible plastic substrates, resulting in screens and displays that can be rolled up; and (3) they are brighter than inorganic LEDs because multiple thin, emissive layers can be deposited on a single device. OLED television screens and mobile phone displays have gained popularity in recent years as thinner and more energy-efficient alternatives to their non-organic counterparts; however, blue OLEDs have been optimized to last for only hundreds of hours, limiting their utility in these applications. Similar to a blue LED, a long-lasting blue OLED must be created before we can realize the full potential of OLEDs in these applications. OLED devices can generate light from purely organic molecules (fluorescent) or organometallic molecules (phosphorescent). Light emission in OLEDs proceeds via recombination of electrons and holes and leads to the formation of 25% singlet excitons and 75% triplet excitons. Fluorescent emitters, usually highly conjugated organic molecules, are unable to access the triplet excitons and lose the remaining energy as heat. To address this issue, thermally activated delayed fluorescence (TADF) has been an emerging type of purely organic OLED that can achieve nearly 100% internal quantum efficiency.2Uoyama H. Goushi K. Shizu K. Nomura H. Adachi C. Nature. 2012; 492: 234-238Crossref PubMed Scopus (4903) Google Scholar To achieve TADF, one must minimize the energy gap between the singlet and triplet states so that the two spin states thermally equilibrate, resulting in a reversible intersystem crossing in which non-radiative triplet excitons can up-convert to radiative singlet excitons. Although researchers have fabricated blue TADF OLED devices with external quantum efficiencies (EQEs) comparable to those of PHOLEDs (∼25%),3Miwa T. Kubo S. Shizu K. Komino T. Adachi C. Kaji H. Sci. Rep. 2017; 7: 284Crossref PubMed Scopus (81) Google Scholar TADF-based OLEDs face difficulty with achieving the deep blue color necessary for display applications, as well as short device lifetimes (∼300 hr at 1,000 cd/m2).4Im Y. Byun S.Y. Kim J.H. Lee D.R. Oh C.S. Yook K.S. Adv. Funct. Mater. 2017; 27: 1603007Crossref Scopus (405) Google Scholar Phosphorescent emitters, typically cyclometallated Pt(II)4Im Y. Byun S.Y. Kim J.H. Lee D.R. Oh C.S. Yook K.S. Adv. Funct. Mater. 2017; 27: 1603007Crossref Scopus (405) Google Scholar, 5Fleetham T.B. Huang L. Klimes K. Brooks J. Li J. Chem. Mater. 2016; 28: 3276-3282Crossref Scopus (108) Google Scholar, 6Fleetham T. Li G. Wen L. Li J. Adv. Mater. 2014; 26: 7116-7121Crossref PubMed Scopus (233) Google Scholar and Ir(III)4Im Y. Byun S.Y. Kim J.H. Lee D.R. Oh C.S. Yook K.S. Adv. Funct. Mater. 2017; 27: 1603007Crossref Scopus (405) Google Scholar, 7Lee J. Chen H.-F. Batagoda T. Coburn C. Djurovich P.I. Thompson M.E. Forrest S.R. Nat. Mater. 2016; 15: 92-98Crossref PubMed Scopus (579) Google Scholar, 8Zhang Y. Lee J. Forrest S.R. Nat. Commun. 2014; 5: 5008Crossref PubMed Scopus (332) Google Scholar, 9Lee J. Jeong C. Batagoda T. Coburn C. Thompson M.E. Forrest S.R. Nat. Commun. 2017; 8: 15566Crossref PubMed Scopus (175) Google Scholar complexes, are able to access triplet excited sates via spin-orbit coupling (SOC), which results in radiative pathways from the excited triplet state to the singlet ground state.3Miwa T. Kubo S. Shizu K. Komino T. Adachi C. Kaji H. Sci. Rep. 2017; 7: 284Crossref PubMed Scopus (81) Google Scholar This process of triplet harvesting involves a quantitative transfer of excitation energy to emitting triplets; therefore, OLEDs with phosphorescent emitters (PHOLEDs) can reach efficiencies up to four times higher than those of OLEDs containing fluorescent emitters. Indeed, PHOLEDs have been experimentally shown to achieve nearly 100% internal efficiency and thus provide an excellent platform. Currently, red and green PHOLEDs are used by major display manufacturers and have device lifetimes upward of 106 hr; however, blue PHOLEDs were only recently optimized to operate efficiently for hundreds of hours at the brightness required for commercial applications—orders of magnitude away from the device lifetime necessary for implementation in screens and displays. Furthermore, the efficiency of blue PHOLEDs dramatically drops when they are operated at high current densities to achieve the brightness necessary for television and lighting applications, mainly as a result of long excited-state lifetimes, which lead to increased bimolecular annihilation. Researchers have both developed sophisticated device architectures8Zhang Y. Lee J. Forrest S.R. Nat. Commun. 2014; 5: 5008Crossref PubMed Scopus (332) Google Scholar and introduced molecular additives in the emissive layer9Lee J. Jeong C. Batagoda T. Coburn C. Thompson M.E. Forrest S.R. Nat. Commun. 2017; 8: 15566Crossref PubMed Scopus (175) Google Scholar to mitigate bimolecular annihilation and increase device longevity, but long-term operational stability still remains a substantial challenge. In this issue of Chem, Wong and co-workers describe a system that addresses some of the current limitations associated with blue PHOLEDs.10Sarma M. Tsai W.-L. Lee W.-K. Chi Y. Wu C.-C. Liu S.-H. Chou P.-T. Wong K.-T. Chem. 2017; 3: 461-476Abstract Full Text Full Text PDF Scopus (73) Google Scholar One of the most well-studied blue emitters for use in PHOLED applications is FIrpic (Figure 1A) because of its sky-blue emission and excellent emission efficiency (Figure 1A). Despite its outstanding photophysical properties, FIrpic has been shown to be electrochemically unstable in PHOLED devices and thus lead to extremely short operational lifetimes with T50 = 2.9 hr (T50 = time to 50% of initial luminescence). In this work, Wong and co-workers re-examined compound MS2, a previously reported pyrimidine-based analog of FIrpic, and found that its extremely high quantum yield (ϕfilm = 0.95) and sub-microsecond excited-state lifetime make it an excellent candidate for use in PHOLED devices (Figure 1A). To further expand on this class of Ir(III) complexes and account for the slightly red-shifted emission of MS2 in relation to that of FIrpic, the authors replaced the picolinate-based ancillary ligand with CF3-containing pyridyl-azolate-based ancillary ligands to yield compounds MS17 and MS19 (Figures 1B and 1C). All three PHOLED devices exhibited excellent EQEmax values of 30%–31%, which are comparable to those of the FIrpic device and near the highest EQE values reported for blue PHOLED devices.4Im Y. Byun S.Y. Kim J.H. Lee D.R. Oh C.S. Yook K.S. Adv. Funct. Mater. 2017; 27: 1603007Crossref Scopus (405) Google Scholar Importantly, the MS2- and MS17-based PHOLED devices demonstrated minimal efficiency roll-off at 1,000 cd/m2 (the relevant brightness for lighting and display applications) with EQE values ranging from 29% to 30%. Even at the very high brightness of 8,000 cd/m2, the EQE values exceeded 25% for both MS2 and MS17, most likely as a result of the short excited-state lifetime of these complexes (0.6–0.8 μs) in relation to those of FIrpic and MS19 (1.0–3.3 μs). Importantly, the PHOLED incorporating MS2 exhibited an impressive operational lifetime of T50 > 2,200 hr—a dramatic improvement of at least one order of magnitude over the many previously reported blue PHOLEDs. In sharp contrast, the devices based on MS17 and MS19 exhibited poor operational stability with T50 values from 0.7 to 3.2 hr. Theoretical analysis suggests that the anomalous device stability for MS2 originates from the relatively large energy gap between the emissive 3MLCT state and non-emissive 3MCd-d states, given that thermal population of 3MCd-d states will weaken metal-ligand bonds and potentially lead to ligand dissociation. Wong and co-workers have described an impressive step toward the development of a blue PHOLED, which could eventually be ready for use in practical applications. Importantly, the authors have achieved this remarkable operational lifetime by using a common device design that does not necessitate reconfiguration of other components. The operational lifetime could probably be increased even more through the use of more-complex device architectures that include graded doping and tandem structures8Zhang Y. Lee J. Forrest S.R. Nat. Commun. 2014; 5: 5008Crossref PubMed Scopus (332) Google Scholar or by the inclusion of excited-state managers into the emissive layer.9Lee J. Jeong C. Batagoda T. Coburn C. Thompson M.E. Forrest S.R. Nat. Commun. 2017; 8: 15566Crossref PubMed Scopus (175) Google Scholar The bluest device in this report has Commision Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.30), still insufficient for use in white lighting and displays. In the future, the color purity must be improved if we are to achieve the deep blue color (CIEy < 0.1) required for such applications while retaining exceptional operational lifetimes—a considerable challenge. To date, blue PHOLEDs have yet to meet the required CIE coordinates of (0.14, 0.08), the standard blue defined by the National Television Standards Committee, although some are approaching this benchmark.6Fleetham T. Li G. Wen L. Li J. Adv. Mater. 2014; 26: 7116-7121Crossref PubMed Scopus (233) Google Scholar, 7Lee J. Chen H.-F. Batagoda T. Coburn C. Djurovich P.I. Thompson M.E. Forrest S.R. Nat. Mater. 2016; 15: 92-98Crossref PubMed Scopus (579) Google Scholar The design of phosphorescent OLED molecules with wide-enough band gaps for blue emission, high quantum yields, suitable lifetimes, and sufficient kinetic stability remains a grand challenge in the field. Nonetheless, work by Wong and co-workers represents a major milestone that can ultimately guide the development of commercially viable blue Ir(III)-based phosphorescent emitters. Anomalously Long-Lasting Blue PhOLED Featuring Phenyl-Pyrimidine Cyclometalated Iridium EmitterSarma et al.ChemSeptember 14, 2017In BriefBlue phosphorescent material with both high efficiency and long durability is a long-standing challenge in the field of organic light-emitting devices (OLEDs). This work describes an anomalously blue phosphorescent OLED featuring the phenyl-pyrimidine cyclometalated iridium emitter, which has nearly unitary emission quantum yield, strong horizontal emitting dipole orientation, and short excited-state lifetime to ensure high electroluminescence efficiency and a long device operation lifetime of T50 > 2,200 hr. Full-Text PDF Open Archive